# Multidigit addition & subtraction

We don’t need no more rappers don’t need no more basketball players, no more football players. We need more thinkers. We need more scientists. We need more managers. We need more mathematicians. We need more teachers.

WE NEED MORE PEOPLE WHO CARE.

Tupac Shakur

# Suggestions & sequences to teach strategies to solve multidigit addition & subtraction

## Introduction

* Overview*

- Introduction
- General instructional method
- Jumps of ten
- Using tens & adjusting
- Making jumps of ten backward
- Using known facts
- Counting on, counting back, adding on or removing
- Subtraction with addition
- Doubles
- Doubles, near doubles with addition, doubles with plus and minus
- Doubles and near doubles with subtraction
- Sliding differences
- Canceling out common amounts
- Decompose, compose and create mental algorithms
- Removing to friendly numbers by counting back
- Adjust to the next friendly number
- Swapping digits
- Commutative property of addition

This page includes instructional suggestions and sample sequences to help develop learner's abilities and fluency to solve addition and subtraction problems mentally,, which everyone is capable of learning and using.

Prior to teaching or learning these strategies it is helpful for learners to have had experiences to develop their spacial abilities. Abilities to visualize the relationships of numbers. For example, relationships of different values, as represented on a hundred chart or number line as well as addition and subtraction with arrow math on a hundred chart. Also beneficial, is numerous experiences with single digit addition and subtraction and understanding the different ways addition and subtraction are represented. Additionally, instruction which has included deconstruction and reconstruction of values to add and subtract is helpful. For example:

*... 7 + 8 ... sample 1*

- can be deconstructed to (5 + 2) + 8;
- rearranged as 5 + (8 +2);
*then 5 + (10); 15*

*... 7 + 8 ... sample 2 *

- can be deconstructed to (5 + 2) + (5 + 3);
- rearranged as (5 + 5) + (2 + 3);
- the (10) + (5); 15

Strategies like these can be taught using a * general instructional sequence* along with numerous examples of different types problems, as described in the following sequences.

## General instructional method

*Planning*:

- Select a strategy for learners to invent and practice.
- Jumps of ten
- Using tens & adjusting
- Making jumps of ten backward
- Using known facts
- Counting on, counting back, adding on or removing
- Subtraction with addition
- Doubles
- Doubles, near doubles with addition, doubles with plus and minus
- Doubles and near doubles with subtraction
- Sliding differences
- Canceling out common amounts
- Decompose, compose and create mental algorithms
- Removing to friendly numbers by counting back
- Adjust to the next friendly number
- Swapping digits
- Commutative property of addition

- This list seems daunting at first, but with experience it can become second nature as a consequence of number literacy.
- Create a list of sample problems. Suggestions for making and using a sample list of problems:
- A sample list should be selected, made, and modified depending on the ability of the learner who is to learn or practice the strategy. Problems in a sequence should be incrementally challenging, but not so hard the learner is unable to be successful using the strategy.

- Display one problem at a time.
- Ask. Solve it.
- Provide adequate silence & wait-time for them to solve it.
- Share different solution methods.
- As you share, write all solutions to encourage critical thinking.
- Discuss all the different strategies used.
- If no one uses the selected strategy, think of
a simpler problem where the strategy could be more easily used. If that
fails then you might say:

Here is a way I have seen others solve it. - Follow up with another problem.
- Ask. Did you use the strategy?
- Continue until they are proficient.
- Conclude by asking. What do you think about the strategy?

*Periodically review*:

To help learners remember and use the different strategies, a spiral approach that periodically returns to each presented strategy unexpectedly is helpful. Periodically randomly presenting a problem from each strategy to review.

The problems in the following strategies are provided as samples, which can be modified depending on th learner's abilities. The order of the problems are not the only order in which strategies can be presented. Depending on the learner's experiences with mathematics and their use of a strategy, the list may vary from the orders presented here.

*Conclusions*:

Decide where to start, what problem to follow each problem with, and when to stop if the learners are not ready to move through a complete list.

## Jumps of ten

*Background*: counting by tens.

- 10, 20, 30, 40, ...
- 15 + 10; 15, 25, 35, 45, ...
- 4, 14, 24, 34, 44, ...
- 26, 36, 46, 56, ...

*Samples*:

Randomly select a number and have students call out ten more:

- 5; 15
- 24; 34

- 8 + 10
- 15 + 10
- 22 + 10
- 34 + 10
- 46 + 10
- 78 + 10
- 14 + 20
- 26 + 30
- 24 + 80
- 112 + 10
- 234 + 10
- 23.5 + 10

## Using tens & adjusting

*Background*: counting by tens, making tens & left overs

*Samples*:

- 15 + 9; 15 +(5 + 4); (15 + 5) + 4; 20 + 4; 24
- 15 + 9; 15 + (10 - 1); (15 + 10) - 1; 25 - 1; 24
- 28 + 44; 28 + (40 + 4); (28 + 40) + 4; 68 + 4; 72
- If no one uses the strategy of ten. Give problems from the jumps of ten list. When they are back on track, return to problems with nine.

- 15 + 9
- 15 + 19
- 28 + 19
- 28 + 32
- 39 + 21
- 28 + 44
- 63 + 10
- 43 + 10
- 123 + 10
- 143 + 100
- 143 + 107
- 138 + 20
- 138 + 23
- 138 + 123

## Making jumps of ten backward

*Background*: counting by tens backwards, reducing or adding to tens with left overs. If no one uses the strategy of ten. Give problems from the jumps of ten list.
When they are back on track, return to problems with nine.

*Samples*:

- 62 - 34;
- 62 - 30 - 4;
- (62 - 30) - 4;
- 32 - 4; 28

- 178 - 39;
- 178 - 30 - 9;
- (178 - 30) - 9;
- 148 - 9;
- (148 - 8) - 1;
- 140 - 1; 139

- 40 - 10
- 40 - 20
- 62 - 10
- 62 - 30
- 62 - 34
- 150 - 30
- 178 - 10
- 178 - 30
- 178 - 39

## Using known facts

*Background*: some basic facts

*Samples*:

- Know: 2 + 2 = 4, ∴ 2 + 3 = 5
- Know: 7 + 7 = 14, ∴ 8 + 6 = 14
- Know: 8 + 6 = 14, ∴ 8 + 7 = 14 + 1
- Know: 9 + 6 = 15, ∴ 8 + 6 = 14)
- Know: 8 + 2 + 5 = 15, ∴ 8 + 7 = 15

Initially help students learn the addition facts they struggle to remember, which an include most of the facts with an addend above five.

- 4 + 7
- 8 + 6
- 14 + 6
- 7 + 7
- 19 + 6
- 18 + 4
- 23 + 5
- 24 + 7
- 36 + 7

## Counting on, counting back, adding on or removing

*Background*: Because addition and subtraction are related so are adding on and removing.

*Samples*:

- 62 - 4
- Makes more sense to remove 4, or work backwards, from 62 to 58.

- 62 - 54
- This makes more sense to add on from 54, 6 + 2 = 8.

- 33 - 4;
- Remove or work back 33 - 3; 30 - 1; 29

- 33 - 7;
- Remove or work back 33 - 3; 30 - 4; 26

- 42 - 37;
- Add on 37 + 3; + 2; 5

- 33 - 28;
- Add on 28 + 2; + 3; 5

*Suggestions*:

When numbers are closer together it is better to add.

When they are far apart it's better to work backward.

Structure your sequences with that in mind to help learn when to add or remove.

- 31 - 3
- 31 - 24
- 42 - 35
- 32 - 28
- 67 - 12
- 67 - 54

## Subtraction with addition

*Background: It is important to learn subtraction problems can be solved
with addition strategies.*

*Samples*:

- 343 - 192
- Start with 192 and jump 8 to 200.
- Find the difference of 200 and 343; 143
- Then add 143 and 8; 151
- 175 - 139
- Start with 139 and jump to 140
- Remember 1;
- Jump to 150; remember 10
- Add 10 to 1; 11 and
- Lastly jump to 175; 11 + 25; 36

- 243 - 43
- 243 - 239
- 243 - 192
- 156 - 40
- 166 - 32
- 986 - 943
- 346 - 193
- 258 - 136
- 894 - 144

## Doubles

Strategy to introduce *doubles* to young children:

- Read Madeline "…In an old house in Paris, that was covered with vines, lived twelve little girls in two straight lines …"
- How many people in each line if in pairs? What if 14? …
- Illustrate skip counting number line with pictures of kids in line two by two.
- Below appropriate kids put 1 + 1, 2 + 2, 3 + 3 and below that put 2, 4, 6, 8.
- Ask. What else comes in twos? Mittens, shoes …
- Ask. What happens when the shoes come off and at door?
- Tell. Draws your family's shoes at the door.
- Double your number.
- Board game where double a roll on a die to move.
- It's like 10 shoes, five left and five right, like 5 pairs or ten shoes.

Randomly say numbers and ask for their double.

- 3, 4, 2, 1, 5, 8, 5, 7, 2, 9, 11, 5, 6, 7, 2, 9, 3, 8, 2, 5, 3, 4, 10
- 13, 8, 6, 9, 4, 22, 12

## Doubles, near doubles with addition, doubles with plus and minus

*Background*: doubles.

*Samples*:

- 3 + 3; double
- 3 + 4; use double plus one
- 4 + 4; double
- 4 + 5; use double plus one
- 5 + 5; double
- 5 + 6; use double plus one
- 5 + 4; use double plus one
- 7 + 7; double
- 7 + 6; use double plus one
- 8 + 8; double
- 8 + 9 use double plus one
- 9 + 9 double
- 8 + 6 near double; 7 + 7
- 12 + 14 near double; 13 + 13
- 12 + 10 near double; 11 + 11
- 25 + 27 near double 26 + 26

- When students understand these, give them problems with 2 more or less
to develop
*near doubles*; - 8 + 6,
*near doubles*to 7 + 7 if subtract 1 from 8 and add it to 7

*Mix them up*:

- 8 + 8
- 8 + 7
- 8 + 9
- 8 + 6
- 12 + 12
- 13 + 12
- 12 + 14
- 12 + 10
- 25 + 25
- 25 + 26
- 25 + 27
- 25 + 24
- 23 + 25
- 18 + 16
- 18 + 17
- 18 + 14

## Doubles and near doubles with subtraction

*Background*: See doubles and near doubles above.

*Samples*:

- 30 - 15; think double 15 is 30; ∴ 30 - 15 is 15
- 52 - 25; think 52 is double 25; ∴ 52 - 25 is 25 + 2

- 50 - 25
- 52 - 25
- 70 - 35
- 72 - 35
- 40 - 20
- 40 - 21
- 40 - 19

## Sliding differences

Adding 51 + 49 by adjusting each to 50 + 50, doesn't work the same way with subtraction.

Subtraction is the difference. 51 - 49. Visualize a number line. The difference between the numbers has to stay the same. Therefore, to keep them the same, a value of adjustment can be added or subtracted to both. For example: to make it easier for some to see, add one to both, or sliding both. 51 and 49; slide one to 52 - 50; which is easier to see a difference of 2.

Another: 52 - 34. Could add 6 to each; 58 - 40, which is easier to see a difference of 18. You can use a number line to model how the difference between 52 and 34 stays the same (18) if the numbers are slid from 52 t0 58 and 34 to 40. can slide in either direction.

This strategy is Not for below third grade. This is a difficult strategy for children to understand. However for problems such as: 1436 - 188, adding 12, makes a much easier problem 1448 - 200,

- 175 - 139
- 174 - 138
- 173 - 137

## Canceling out common amounts

Is the same as *sliding differences*. 120 - 109, cancel 100 from both
makes 20 - 9; 11.

This can be modeled with a double number line to show that the difference between the numbers stay the same as the numbers are slid along the number line. Which, is the same as canceling the 100's from each to leave 20 - 9; 11 or the difference between 20 and 9 is 11. The same difference between 120 and 109.

20342 - 10012; 10330

## Decompose, compose and create mental algorithms

Children usually decompose
numbers* left to right* by place values.

28 + 44 is decomposed into

- 20 + 40; 60
- 8 + 4; 12
- 60 + 12; 72

Learners will solve problems by decomposing by place values. It takes a lot of experience before the are able to understand and operationalize addition and subtraction with two or more digits, each with different place values. Eventually, they will understand to add 23 and 34, they must operate on all numbers within their place values:

- 23 + 34; decompose
- (20 + 3) + (30 + 4); regroup
- (20 + 30) + (3 + 4); add the tens,
- 50 + (3 + 4); add the ones
- 50 + 7; then combine the tens and the ones; 57

To help learners move to a more efficient strategy, you may need to encourage them not to decompose both numbers, but to select one to decompose and one to add on from.

- 23 + 34; decompose 23 and don't decompose 34
- (20 + 3) + 34; commutative property
- (34 + 20) + 3; add
- 54 + 3; 57

- 78 + 26; decompose 26 and don't decompose 78
- 78 + (20 + 6)
- (78 + 20) + 6 add
- 98 + 6; 104

- 183 + 58; decompose 58 and don't decompose 123
- 183 + (50 + 8); commutative property
- (183 + 50) + 8; add [think (183 + 20 + 30); (203 + 30)
- 233 + 8; add [think 233 + 7 + 1]
- 241

In today's world with calculators and spread sheets, the strategies presented here can be used as mental algorithms using mostly the strategies of decomposition, composition, and working from

left to right!Together they can provide all a person needs for everyday calculations.

The beauty of the traditional algorithm isn't in using it over other strategies.

It is in its efficiency and its elegance of working in all situations, particularly in advanced mathematics.

## Removing to friendly numbers by counting back

*Background*: friendly numbers are tens and fives

*Sample*:

Subtract parts by removing amounts to work with friendly numbers.

- 143 - 24;
- think remove move 3 to 140
- 143 - 3; leaves

- 140 - 21;
- then think to remove 20
- 140 - 20; leaves

- 120 - 1; 119

- 164 - 25
- 182 - 43

## Adjust to the next friendly number

*Background*: friendly numbers are tens and fives

*Samples*:

- 47 + 4;
- adjust to 50
- 50 + 1; 51

- 38 + 6; adjust to 40
- 40 + 4; 44

- 98 + 37; adjust to 100
- 100 + 35; 135

- 27 + 49; adjust to 50
- 26 + 50; 76

- 36 + 118; adjust to 120
- 34 + 120; 154

- 227 + 164; adjust to 230
- 230 + 161; 391

- 47 + 24; adjust to 50
- 50 + 21; 71

- 47 + 34; adjust to 50
- 50 + 31; 81

- 98
+ 29; adjust to 100
- 100 + 27; 127 Or adjust to 30; 97 + 30; 127

- 96 + 29; adjust to 30
- 95 + 30; 125

- 298 + 37 adjust to 300
- 300 + 35; 335

- 442 + 199 adjust to 200
- 441 + 200; 641

- 48 + 4
- 36 + 6
- 88 + 7
- 99 + 6
- 47 + 33
- 42 + 58
- 46 + 33
- 118 + 83

## Swapping digits

*Background*: This is one that probably will need modeling.

*Samples*:

- 2
*9*3 + 9*1*9; swap digits in the same place value (tens)- 213 + 999; then use adjust (add 1 to 999 and subtract one from 213)
- 212 + 1000 = 1 212; add

- 3
*4*+ 1*9*swap digits (ones)- 39 + 14; then use adjust (add 1 to 39 and subtract one from 14)
- 39 +1 + 13 = 53; add
- Hard to argue using tens and adjusting is not easier (34 + 20) - 1; 53

Good to prove learning is working if it is mentioned. ☺

- 71 + 26 swap digits
- 76 + 21; then adjust
- 76 + 1 + 20; add
- 97

- 449 + 192 swap digits
- 499 + 142; then adjust
- 500 + 141; 641

- 53 + 29
- 82 + 78
- 84 + 78
- 129 + 93
- 449 + 291

## Commutative property of addition

*Background*: Commutative property of addition. Commutative property of addition is the order in which a pair of addends is added does not affect the sum. For all real numbers a and b, a + b = b + a

*Samples*:

- 8 + 7 = 15, ∴ 7 + 8 = 15
- 9 + 8 = 17, ∴ 8 + 9 = 17
- 8 + 6 = 14, ∴ 6 + 8 = 14